Electromagnetic interference (EMI), sometimes called radiofrequency interference (RFI), may be generated by any change in an electrical current that passes through a device or a circuit. The extent of the interference is a function of the amplitude of the change in the current. Conducted noise, which is a form of interference that may be generated within electrical equipment such as rotating electrical machines, results from unwanted conducted voltages and currents that travel through input or output lines, control leads or power conductors.
Direct current (DC) motors and engines may generate conducted noise in any number of ways. For example, voltage transients that are created as brushes slide across a commutator bar, or as brushes transfer from one commutator bar to another, may generate interference particularly in low frequency ranges. Moreover, current ripples created by commutation or other sources may create an undesirable audio-frequency hum.
Many existing power supplies are designed to maintain a steady output voltage and to filter out high frequency noise. Low frequency noise, such as that which is generated by the commutation of coils of a DC motor or engine, is not easily filtered by existing systems and methods, however. For this reason, many common low frequency conducted noise sources, such as ripples in DC voltage or current, are generally permitted to transfer through power supplies to components. Problems associated with EMI or RFI in general, and with conducted noise in particular, may become more and more prevalent as electronic components shrink in size, operate at high power, or utilize computing devices that are clocked at high frequencies.
Recently implemented electrical infrastructure requirements, such as those set forth in Section 10 of the GR-1089-CORE Network Equipment-Building System (NEBS) standards, have limited the amount of noise that may be generated in the low frequency range commonly known as the “voice band,” which extends approximately from 300 Hz to 3400 Hz and is utilized for voice transmission and/or data communications.
Many systems and methods have attempted to address the problems associated with conducted noise in DC components. One such solution is to install decoupling capacitors in close proximity to the offending DC component, such as a fan installed into a fan tray with a local fan controller printed circuit board. Installing decoupling capacitors near the controller board minimizes the lengths of the wires between the fan motor and the capacitors, thereby minimizing their natural inductance and improving the filtering performance of the capacitors. However, it is not always possible to install capacitors adjacent to components, especially in miniature electronic circuitry. If the capacitors cannot be installed near the DC equipment, long wires—which can create very inductive paths that require more elaborate filters—may be required to connect the capacitors to the equipment.
Another solution is to install a passive inductor-capacitor (LC) combination filter in series with the offending DC component. However, in order to filter conducted noise from the low frequency voice band, the inductors and/or capacitors in an LC combination filter must often be very large and may require complex materials and geometries which can greatly increase the cost of such systems, and may further complicate their installation into equipment.